This is paragraph style, the default style for your page content.

this is h1 style, a good idea to use h1 style on the first sentence or two on every page, using your keywords (google sees the h1 tag as important).

this is h2 style

this is h3 style – the style used for the page titles, and article/blog titles

this is h4 style

this is h5 style
this is h6 style